Re: [RFC 1/2 -tip/master] x86, x2apic: minimize IPI register writes using cluster groups

Cyrill Gorcunov
2011-02-14T15:10:07Z

On 02/14/2011 02:45 PM, Ingo Molnar wrote:
>
> * Cyrill Gorcunov<gorcunov@gmail.com> wrote:
>
>> In the case of x2apic cluster mode we can group IPI register writes based on the
>> cluster group instead of individual per-cpu destiantion messages. This reduces the
>> apic register writes and reduces the amount of IPI messages (in the best case we
>> can reduce it by a factor of 16).
>>
>> With this change, microbenchmark measuring the cost of flush_tlb_others(), with
>> the flush tlb IPI being sent from a cpu in the socket-1 to all the logical cpus in
>> socket-2 (on a Westmere-EX system that has 20 logical cpus in a socket) is 3x
>> times better now (compared to the former 'send one-by-one' algorithm).
>
> Pretty nice!
>
> I have a few structural and nitpicking comments:

Thanks a lot for review, Ingo! I'll address all the nits during this week.

...
>
>> +void x2apic_init_cpu_notifier(void)
>> +{
>> + int cpu = smp_processor_id();
>>
>> + zalloc_cpumask_var(&per_cpu(cpus_in_cluster, cpu), GFP_KERNEL);
>> + zalloc_cpumask_var(&per_cpu(ipi_mask, cpu), GFP_KERNEL);
>> + BUG_ON(!per_cpu(cpus_in_cluster, cpu) || !per_cpu(ipi_mask, cpu));
>
> Such a BUG_ON() is not particularly user friendly - and this could trigger during
> CPU hotplug events, i.e. while the system is fully booted up, right?
>
> Thanks,
>
> Ingo

Yup is not that much friendly but it's called during system bootup,
hotplug events are handled by

+static int __cpuinit
+cluster_setup(struct notifier_block *nfb, unsigned long action, void *hcpu)
+{
+ unsigned int cpu = (unsigned long)hcpu;
+ int err = 0;
+
+ switch (action) {
+ case CPU_UP_PREPARE:
+ zalloc_cpumask_var(&per_cpu(cpus_in_cluster, cpu), GFP_KERNEL);
+ zalloc_cpumask_var(&per_cpu(ipi_mask, cpu), GFP_KERNEL);
+ if (!per_cpu(cpus_in_cluster, cpu) || !per_cpu(ipi_mask, cpu)) {
+ free_cpumask_var(per_cpu(cpus_in_cluster, cpu));
+ free_cpumask_var(per_cpu(ipi_mask, cpu));
+ err = -ENOMEM;
+ }
+ break;

so it returns -ENOMEM if failed. And btw just noted that we forgot to make
x2apic_init_cpu_notifier being in __init section.

Or I miss something?

--
Cyrill